Solid-state logarithmic image sensing device

ABSTRACT

In a solid-state image-sensing device, after completion of image sensing by individual pixels, in each pixel, a signal φVRB fed to a capacitor C 1  connected to the gate of a first MOS transistor T 1  is turned to a high level to make it easy for a negative electric charge to flow into the MOS transistor T 1 . This permits quick recombination of the positive electric charges accumulated at the drain and gate of the MOS transistor T 1 , at the gate of a MOS transistor T 2 , at the anode of a photodiode, and in a capacitor C 2 , and thereby makes quick resetting possible.

This application is based on applications Nos. H11-207696 and H11-208267both filed in Japan on Jul. 22, 1999, the entire contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image-sensing apparatus, andparticularly to an image-sensing apparatus incorporating a solid-stateimage-sensing device having a plurality of pixels arranged therein.

2. Description of the Prior Art

Solid-state image-sensing devices designed for use in image-sensingapparatuses are not only small-sized, light-weight, and power-saving,but also free from image distortion, free from image burn-in, andresistant to unfavorable environmental conditions such as vibration andelectric fields. Moreover, such image-sensing devices can bemanufactured by a process common or analogous to one used to manufactureLSIs (large-scale integrated circuits), and therefore they are reliableand fit for mass production. For these reasons, solid-stateimage-sensing devices having pixels arranged in a line are widely usedin facsimile machines, flat-bed scanners, and the like, and solid-stateimage-sensing devices having pixels arranged in a matrix are widely usedin video cameras, digital cameras, and the like. Solid-stateimage-sensing devices are roughly grouped into two types according tothe means they use to read out (extract) the photoelectric chargesgenerated in their photoelectric conversion devices, namely a CCD typeand a MOS type. A CCD-type image-sensing device accumulatesphotoelectric charges in potential wells while transferring them, andhas the disadvantage of a comparatively narrow dynamic range. On theother hand, a MOS-type image-sensing device reads out electric chargesaccumulated in the pn-junction capacitances of photodiodes through MOStransistors.

Now, how each pixel is configured in a conventional MOS-type solid-stateimage-sensing device will be described with reference to FIG. 37. Asshown in this figure, a photodiode PD has its cathode connected to thegate of a MOS transistor T101 and to the source of a MOS transistorT102. The MOS transistor T101 has its source connected to the drain of aMOS transistor T103, and this MOS transistor T103 has its sourceconnected to an output signal line VOUT. A direct-current voltage VPD isapplied to the drain of the MOS transistor T101 and to the drain of theMOS transistor T102, and a direct-current voltage VPS is applied to theanode of the photodiode.

When light enters the photodiode PD, a photoelectric charge is generatedtherein, and this electric charge is accumulated at the gate of the MOStransistor T101. Here, when a pulse signal φV is fed to the gate of theMOS transistor T103 to turn this MOS transistor T103 on, a currentproportional to the electric charge accumulated at the gate of the MOStransistor T101 flows through the MOS transistors T101 and T103 to theoutput signal line. In this way, it is possible to read an outputcurrent that is proportional to the amount of incident light. After thissignal has been read, the MOS transistor T103 is turned off and therebythe MOS transistor T102 is turned on so that the gate voltage of the MOStransistor T101 will be initialized.

As described above, in a conventional MOS-type solid-state image-sensingdevice, at each pixel, the photoelectric charge generated in thephotodiode PD and then accumulated at the gate of the MOS transistorT101 is directly read out. This, however, leads to a narrow dynamicrange and thus demands accurate control of the amount of exposure.Moreover, even if the amount of exposure is controlled accurately, theobtained image tends to suffer from flat blackness in dim portionsthereof and saturation in bright portions thereof.

On the other hand, the assignee of the present invention has onceproposed a solid-state image-sensing device including a light-sensingmeans that generates a photoelectric current in accordance with theamount of incident light, a MOS transistor to which the generatedphotoelectric current is fed, and a bias-supplying means that supplies abias to the MOS transistor to bring it into a state in which asubthreshold current flows therethrough so that the photoelectriccurrent is subjected to logarithmic conversion (refer to U.S. Pat. No.5,241,575).

In this solid-state image-sensing device, when the individual pixels arereset to their original state after image sensing, except in alow-brightness region, a current (called a reset current) having theopposite polarity to the photoelectric current readily flows into theMOS transistor. Therefore, it is easy for the photoelectric chargeaccumulated in the MOS transistor to recombine with the reset current,and thus resetting is achieved quickly. However, in a low-brightnessregion, under the influence of the threshold voltage of the MOStransistor, the reset current has difficulty in flowing into the MOStransistor. Therefore, it is difficult for the photoelectric chargeaccumulated in the MOS transistor to recombine with the reset current,and thus resetting takes an unduly long time. In this way, in alow-brightness region, the individual pixels exhibit poor response, andthis causes an after-image to appear in immediately subsequent imagesensing.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an image-sensingapparatus that permits shooting of a subject having a wide brightnessrange, ranging from a high-brightness region to a low-brightness region,with high resolution and that permits individual pixels to be reset totheir original state quickly even in a low-brightness region and therebyoffers quick response.

To achieve the above object, according to one aspect of the presentinvention, an image-sensing apparatus is provided with a photoelectricconversion portion having a photosensitive element producing an electricsignal in accordance with amount of incident light and a firsttransistor having a first electrode connected to the photosensitiveelement, a second electrode, and a control electrode. This photoelectricconversion portion outputs the electric signal output from thephotosensitive element and converted natural-logarithmically by makingthe first transistor operate in a subthreshold region. The image-sensingapparatus is further provided with a lead-out path for feeding theelectric signal output from the photoelectric conversion portion to anoutput signal line and a controller for resetting the potential in thefirst transistor by switching the voltage applied to the controlelectrode of the first transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of the present invention will becomeclear from the following description, taken in conjunction with thepreferred embodiments with reference to the accompanying drawings inwhich:

FIG. 1 is a block diagram showing the internal configuration of animage-sensing apparatus according to the invention;

FIG. 2 is a block circuit diagram illustrating the overall configurationof a two-dimensional solid-state image-sensing device embodying theinvention;

FIG. 3 is a circuit diagram showing the configuration of each pixel in afirst embodiment of the invention;

FIG. 4 is a timing chart of the signals fed to each pixel in the firstembodiment;

FIGS. 5A to 5E are diagrams showing the structure of and the potentialrelationship observed in a pixel according to the invention;

FIG. 6 is a circuit diagram showing the configuration of each pixel in asecond embodiment of the invention;

FIG. 7 is a timing chart of the signals fed to each pixel in the secondembodiment;

FIGS. 8A to 8E are diagrams showing the structure of and the potentialrelationship observed in a pixel according to the invention;

FIG. 9 is a block circuit diagram illustrating the overall configurationof another two-dimensional solid-state image-sensing device embodyingthe invention;

FIGS. 10A and 10B are circuit diagrams of a portion of the circuit shownin FIG. 9;

FIG. 11 is a circuit diagram showing the configuration of each pixel ina third embodiment of the invention;

FIG. 12 is a timing chart of the signals fed to each pixel in the thirdembodiment;

FIG. 13 is a circuit diagram showing the configuration of each pixel ina fourth embodiment of the invention;

FIG. 14 is a circuit diagram showing the configuration of each pixel ina fifth embodiment of the invention;

FIG. 15 is a circuit diagram showing the configuration of each pixel ina sixth embodiment of the invention;

FIG. 16 is a timing chart of the signals fed to each pixel in the sixthembodiment;

FIGS. 17A to 17C are diagrams showing the potential relationshipobserved during variation detection;

FIG. 18 is a circuit diagram showing the configuration of each pixel ina seventh embodiment of the invention;

FIG. 19 is a circuit diagram showing the configuration of each pixel inan eighth embodiment of the invention;

FIG. 20 is a timing chart of the signals fed to each pixel in the eighthembodiment;

FIG. 21 is a circuit diagram showing the configuration of each pixel ina ninth embodiment of the invention;

FIG. 22 is a circuit diagram showing the configuration of each pixel ina tenth embodiment of the invention;

FIG. 23 is a block circuit diagram illustrating the overallconfiguration of a two-dimensional solid-state image-sensing deviceembodying the invention, in a case where the active elements within apixel are composed of P-channel MOS transistors;

FIG. 24 is a circuit diagram showing the configuration of each pixel inan eleventh embodiment of the invention;

FIG. 25 is a circuit diagram showing the configuration of each pixel ina twelfth embodiment of the invention;

FIG. 26 is a block circuit diagram illustrating the overallconfiguration of another two-dimensional solid-state image-sensingdevice embodying the invention, in a case where the active elementswithin a pixel are composed of P-channel MOS transistors;

FIGS. 27A and 27B are circuit diagrams of a portion of the circuit shownin FIG. 26;

FIG. 28 is a circuit diagram showing the configuration of each pixel ina thirteenth embodiment of the invention;

FIG. 29 is a circuit diagram showing the configuration of each pixel ina fourteenth embodiment of the invention;

FIG. 30 is a circuit diagram showing the configuration of each pixel ina fifteenth embodiment of the invention;

FIG. 31 is a circuit diagram showing the configuration of each pixel ina sixteenth embodiment of the invention;

FIG. 32 is a circuit diagram showing the configuration of each pixel ina seventeenth embodiment of the invention;

FIG. 33 is a circuit diagram showing the configuration of each pixel inan eighteenth embodiment of the invention;

FIG. 34 is a circuit diagram showing the configuration of each pixel ina nineteenth embodiment of the invention;

FIG. 35 is a circuit diagram showing the configuration of each pixel ina twentieth embodiment of the invention;

FIG. 36 is a block diagram showing the internal configuration of animage input apparatus provided with a solid-state image-sensing devicehaving pixels configured according to one of the embodiments of theinvention; and

FIG. 37 is a circuit diagram showing the configuration of each pixel ina conventional solid-state image-sensing device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Image-Sensing Apparatus

First, an image-sensing apparatus according to the present inventionwill be described with reference to the drawings. FIG. 1 is a blockdiagram showing the internal configuration of an image-sensing apparatusaccording to the invention. The image-sensing apparatus 52 shown in FIG.1 includes the following components. A solid-state image-sensing device1 receives the light from a subject through an objective lens 51. Acontroller 100 controls the operation of the solid-state image-sensingdevice 1. A processor 101 processes the signal output from thesolid-state image-sensing device 1.

In this image-sensing apparatus 52 configured as described above, thecontroller 100, by switching the voltage of a signal φVPS or φVRB,described later, fed to the individual pixels provided in thesolid-state image-sensing device 1, switches the operation of theindividual pixels between image sensing and resetting. Moreover, thecontroller 100, by feeding a pulse signal φV to the individual pixels,makes them feed the electric signals obtained as a result ofphotoelectric conversion to the processor 101. The electric signal thusfed to the processor 101 is processed by the processor 101 and is thenfed to the succeeding circuit.

In addition, in a sixth, seventh, sixteenth, and seventeenth embodimentdescribed later, the solid-state image-sensing device 1 is controlled bythe controller 100 by being fed with various signals as mentioned above.The output of the solid-state image-sensing device 1 is fed to theprocessor 101, and the processor 101 checks, on the basis of the timingwith which the output is yielded, whether the output is image data orcompensation data, and then outputs it to the succeeding stage. At thistime, to permit the controller 100 to recognize whether the signalcurrently being fed to the succeeding stage is image data orcompensation data, the processor 101 feeds the controller 100 withinformation on the signal it is currently outputting.

The above-described configuration and operation of the image-sensingapparatus are common to all of the descriptions to follow. Now, thesolid-state image-sensing device provided in this image-sensingapparatus will be described.

First Example of the Pixel Configuration

Hereinafter, solid-state image-sensing devices embodying the presentinvention will be described with reference to the drawings. FIG. 2schematically shows the configuration of part of a two-dimensionalMOS-type solid-state image-sensing device embodying the invention. Inthis figure, reference symbols G11 to Gmn represent pixels that arearranged in a two-dimensional array (in a matrix). Reference numeral 2represents a vertical scanning circuit, which scans lines (rows) 4-1,4-2, . . . , 4-n sequentially. Reference numeral 3 represents ahorizontal scanning circuit, which reads out, sequentially pixel bypixel in a horizontal direction, the signals fed from the individualpixels to output signal lines 6-1, 6-2, . . . , 6-m as a result ofphotoelectric conversion performed in those pixels. Reference numeral 5represents a power line. The individual pixels are connected not only tothe lines 4-1, 4-2, . . . , 4-n, to the output signal lines 6-1, 6-2, .. . , 6-m, and to the power line 5 mentioned above, but also to otherlines (for example clock lines and bias supply lines). These otherlines, however, are omitted in FIG. 2, and are shown in FIGS. 3 and 6,which show a first and a second embodiment, respectively, of theinvention.

As shown in FIG. 2, for each of the output signal lines 6-1, 6-2, . . ., 6-m, one N-channel MOS transistor Q2 is provided. Here, a descriptionwill be given only with respect to the output signal line 6-1 as theirrepresentative. The MOS transistor Q2 has its drain connected to theoutput signal line 6-1, has its source connected to a signal line 9serving as a final destination line, and has its gate connected to thehorizontal scanning circuit 3. As will be described later, within eachpixel, another N-channel MOS transistor (a third MOS transistor) T3functioning as a switch is provided. Whereas this MOS transistor T3serves to select a row, the MOS transistor Q2 serves to select a column.

First Embodiment

Now, a first embodiment (FIG. 3) of the invention, which is applicableto each pixel of the first example of the pixel configuration shown inFIG. 2, will be described with reference to the drawings.

In FIG. 3, a pn photodiode PD serves as a photosensitive element(photoelectric conversion portion). The anode of this photodiode PD isconnected to the gate and drain of a first MOS transistor T1, and alsoto the gate of a second MOS transistor T2. The source of the MOStransistor T2 is connected to the drain of a third MOS transistor T3 forselecting a row. The source of this MOS transistor T3 is connected tothe output signal line 6 (this output signal line 6 corresponds to theoutput signal lines 6-1, 6-2, . . . , 6-m in FIG. 2). The MOStransistors T1 to T3 are all N-channel MOS transistors, and have theirback gates grounded.

A direct-current voltage VPD is applied to the cathode of the photodiodePD. On the other hand, a direct-current voltage VPS is applied to thesource of the MOS transistor T1, and also to one end of a capacitor C2of which the other end is connected to the source of the MOS transistorT2. A signal φD is fed to the drain of the MOS transistor T2, and asignal φV is fed to the gate of the MOS transistor T3. Moreover, thenode between the gates of the MOS transistors T1 and T2 is connected toone end of a capacitor C1, and a signal φVRB is fed to the other end ofthis capacitor C1. The signal φVRB is a binary signal that takes one oftwo predetermined levels, i.e. either a low level or a high level, at atime. Here, a low level refers to a level equal to, for example, theground level, and a high level refers to a voltage that is higher thanthat.

(1) How to Convert the Light Incident on the Individual Pixels into anElectric Signal

In this pixel having the circuit configuration shown in FIG. 3, topermit the MOS transistor T1 to operate in a subthreshold region, thesignal φVRB fed to the capacitor C1 is turned to a low level. When thesignal φVRB is turned to a low level in this way, the capacitor C1 actsin a manner quite similar to the capacitors formed by insulating oxidefilms at the gate and back gate of the MOS transistors T1 and T2.

In this state, when light enters the photodiode PD, a photoelectriccurrent is generated therein, and, due to the subthresholdcharacteristics of MOS transistors, a voltage having the value obtainedby converting the photoelectric current natural-logarithmically appearsat the gates of the MOS transistors T1 and T2. This voltage causes acurrent to flow through the MOS transistor T2, and, as a result, anelectric charge that is equivalent to the value obtained by convertingthe integral of the photoelectric current natural-logarithmically isaccumulated in the capacitor C2. That is, a voltage proportional to thevalue obtained by converting the integral of the photoelectric currentnatural-logarithmically appears at the node “a” between the capacitor C2and the source of the MOS transistor T2. Here, the MOS transistor T3 isoff.

Next, the pulse signal φV is fed to the gate of the MOS transistor T3 toturn this MOS transistor T3 on. This causes the electric chargeaccumulated in the capacitor C2 to be fed as the output current to theoutput signal line 6. The current thus fed to the output signal line 6has the value obtained by converting the integral of the photoelectriccurrent natural-logarithmically. In this way, it is possible to read asignal (output current) that is proportional to the logarithm of theamount of incident light. After this signal has been read, the MOStransistor T3 is turned off.

(2) How to Reset the Individual Pixels

Now, how resetting is achieved in the pixel having the circuitconfiguration shown in FIG. 3 will be described with reference to thedrawings. FIG. 4 is a timing chart of the signals fed to the signallines connected to the individual circuit elements constituting eachpixel during resetting. FIG. 5A is a sectional view showing thestructure of the photodiode PD and the MOS transistor T1. FIGS. 5B to 5Eare diagrams showing the relationship among the potentials at variousparts of the sectional view shown in FIG. 5A. In FIGS. 5B to 5E, thearrow at the left indicates the direction in which the potentialincreases.

Incidentally, as shown in FIG. 5A, the photodiode PD is formed, forexample, by forming an N-type well layer 11 in a P-type semiconductorsubstrate (hereinafter referred to as the “P-type substrate”) 10 andthen forming, in this N-type well layer 11, a P-type diffusion layer 12.On the other hand, the MOS transistor T1 is formed by forming N-typediffusion layers 13 and 14 in the P-type substrate 10 and then forming,on top of the channel left between those N-type diffusion layers 13 and14, an oxide film 15 and, further on top thereof, a polysilicon layer16. Here, the N-type well layer 11 functions as the cathode of thephotodiode PD, and the P-type diffusion layer 12 functions as the anodethereof. On the other hand, the N-type diffusion layers 13 and 14function as the drain and the source, respectively, of the MOStransistor T1, and the oxide film 15 and the polysilicon layer 16function as the gate insulating film and the gate electrode,respectively, thereof. Here, in the P-type substrate 10, the regionbetween the N-type diffusion layers 13 and 14 is called the sub-gateregion.

As described under (1) above, in the pixel having the circuitconfiguration shown in FIG. 3, when the pulse signal φV is fed to thegate of the MOS transistor T3, an electric signal (output signal)obtained by converting the amount of incident light logarithmically isoutput to the output signal line 6. After this output signal has beenoutput, the pulse signal φV turns to a low level, and resetting starts.Now, how this resetting is achieved will be described with reference toFIGS. 4 and 5A to 5E.

As described above, after the output signal has been output as a resultof the pulse signal φV being fed to the gate of the MOS transistor T3,resetting starts. When resetting starts, a negative electric chargeflows into the MOS transistor T1 via the source thereof, so that thepositive electric charge accumulated at the gate and drain of the MOStransistor T1, at the gate of the MOS transistor T2, at the anode of thephotodiode PD, and in the capacitor C1 recombine therewith. As a result,as shown in FIG. 5B, the potentials at the drain and sub-gate region ofthe MOS transistor T1 drop down to certain levels.

In this way, the potentials at the drain and sub-gate region of the MOStransistor T1 start being reset to their original levels, but the speedof resetting becomes slow when those potentials reach certain levels.This is particularly noticeable when a thus far brightly-lit subject hassuddenly become dimly-lit. To avoid this, next, the voltage φVRB fed tothe capacitor C1 is made higher so as to make the gate voltage of theMOS transistor T1 higher. By making the gate voltage of the MOStransistor T1 higher in this way, the potentials in the MOS transistorT1 change so as to have a relationship as shown in FIG. 5C; that is, thepotentials at the sub-gate region and drain of the MOS transistor T1become higher. This increases the amount of negative electric chargethat flows into the MOS transistor T1 via the source thereof, andthereby permits quick recombination of the positive electric chargesaccumulated at the gate and drain of the MOS transistor T1, at the gateof the MOS transistor T2, at the anode of the photodiode PD, and in thecapacitor C1.

As a result, as shown in FIG. 5D, the potentials at the drain andsub-gate region of the MOS transistor T1 become lower than in the stateshown in FIG. 5C. When the potentials in the MOS transistor T1 havechanged so as to have a relationship as shown in FIG. 5D, the voltageφVRB fed to the capacitor C1 is turned to a low level, and the gatevoltage of the MOS transistor T1 is lowered. This causes the potentialsat the drain and sub-gate region of the MOS transistor T1 to have arelationship as shown in FIG. 5E, and thus causes those potentials to bereset to their original levels. After the potentials in the MOStransistor T1 have been reset to their original levels in this way, thevoltage of the signal φD is turned to a low level so that the capacitorC2 is discharged and thereby the potential at the node “a” is reset toits original level. Thereafter, the voltage of the signal φD is turnedback to a high level in preparation for image sensing.

In this way, resetting is achieved by switching the voltage at the gateof the MOS transistor T1 of which the drain is connected to thephotodiode serving as a photosensitive element. This helps improve theresponse of the individual pixels of the solid-state image-sensingdevice, and thereby makes it possible to perform satisfactory imageshooting free from after-images even when a dimly-lit subject is shot orwhen a thus far brightly-lit subject has suddenly become dimly-lit.

Second Embodiment

Next, a second embodiment of the invention will be described withreference to the drawings. FIG. 6 is a circuit diagram showing theconfiguration of each pixel of the solid-state image-sensing device ofthis embodiment. It is to be noted that such elements, signal lines, andothers as are used for the same purposes here as in the pixel shown inFIG. 3 are identified with the same reference symbols, and theirdetailed descriptions will be omitted.

As shown in FIG. 6, in this embodiment, a signal φVPS is fed to thesource of the MOS transistor T1, and the capacitor C1 is omitted. Inother respects, each pixel of this embodiment is configured in the samemanner as in the first embodiment (FIG. 3). The signal φVPS is a binarysignal that takes one of two predetermined levels, i.e. either a lowlevel or a high level, at a time. Here, a high level refers to a voltagethat is approximately equal to the direct-current voltage VPS and thatcauses the MOS transistor T1 to operate in a subthreshold region, and alow level refers to a voltage that is lower than that and that bringsthe MOS transistor T1 into a conducting state.

(1) How to Convert the Light Incident on the Individual Pixels into anElectric Signal

In this pixel having the circuit configuration shown in FIG. 6, topermit the MOS transistor T1 to operate in a subthreshold region, thesignal φVPS fed to the source of the MOS transistor T1 is turned to ahigh level. In this state, when light enters the photodiode PD, aphotoelectric current is generated therein, and, due to the subthresholdcharacteristics of MOS transistors, a voltage having the value obtainedby converting the photoelectric current natural-logarithmically appearsat the gates of the MOS transistors T1 and T2. This voltage causes acurrent to flow through the MOS transistor T2, and, as a result, anelectric charge that is equivalent to the value obtained by convertingthe integral of the photoelectric current natural-logarithmically isaccumulated in the capacitor C2. That is, a voltage proportional to thevalue obtained by converting the integral of the photoelectric currentnatural-logarithmically appears at the node “a” between the capacitor C2and the source of the MOS transistor T2. Here, the MOS transistor T3 isoff.

Next, the pulse signal φV is fed to the gate of the MOS transistor T3 toturn this MOS transistor T3 on. This causes the electric chargeaccumulated in the capacitor C2 to be fed as the output current to theoutput signal line 6. The current thus fed to the output signal line 6has the value obtained by converting the integral of the photoelectriccurrent natural-logarithmically. In this way, it is possible to read asignal (output current) that is proportional to the logarithm of theamount of incident light. After this signal has been read, the MOStransistor T3 is turned off.

(2) How to Reset the Individual Pixels

Now, how resetting is achieved in the pixel having the circuitconfiguration shown in FIG. 6 will be described with reference to thedrawings. FIG. 7 is a timing chart of the signals fed to the signallines connected to the individual circuit elements constituting eachpixel during resetting. FIG. 8A is a sectional view, like FIG. 5A,showing the structure of the photodiode PD and the MOS transistor T1.FIGS. 8B to 8E are diagrams showing the relationship among thepotentials at various parts of the sectional view shown in FIG. 8A. InFIGS. 8B to 8E, the arrow at the left indicates the direction in whichthe potential increases. It is to be noted that no detailed descriptionwill be given of FIG. 8A, which is similar to FIG. 5A described inconnection with the first embodiment.

As described under (1) above, in the pixel having the circuitconfiguration shown in FIG. 6, when the pulse signal φV is fed to thegate of the MOS transistor T3, an electric signal (output signal)obtained by converting the amount of incident light logarithmically isoutput to the output signal line 6. After this output signal has beenoutput, the pulse signal φV turns to a low level, and resetting starts.Now, how this resetting is achieved will be described with reference toFIGS. 7 and 8A to 8E.

As described above, after the output signal has been output as a resultof the pulse signal φV being fed to the gate of the MOS transistor T3,resetting starts. When resetting starts, a negative electric chargeflows into the MOS transistor T1 via the source thereof, so that thepositive electric charges accumulated at the gate and drain of the MOStransistor T1, at the gate of the MOS transistor T2, and at the anode ofthe photodiode PD recombine therewith. As a result, as shown in FIG. 8B,the potentials at the drain and sub-gate region of the MOS transistor T1are reset to a certain degree and thus drop down to certain levels.

In this way, the potentials at the drain and sub-gate region of the MOStransistor T1 start being reset to their original levels, but the speedof resetting becomes slow when those potentials reach certain levels.This is particularly noticeable when a thus far brightly-lit subject hassuddenly become dimly-lit. To avoid this, next, the signal φVPS fed tothe source of the MOS transistor T1 is turned to a low level. Bylowering the source voltage of the MOS transistor T1 in this way, thepotentials in the MOS transistor T1 change so as to have a relationshipas shown in FIG. 8C. This increases the amount of negative electriccharge that flows into the MOS transistor T1 via the source thereof, andthereby permits quick recombination of the positive electric chargesaccumulated at the gate and drain of the MOS transistor T1, at the gateof the MOS transistor T2, and at the anode of the photodiode PD.

As a result, as shown in FIG. 8D, the potentials at the drain andsub-gate region of the MOS transistor T1 become lower than in the stateshown in FIG. 8C. When the potentials in the MOS transistor T1 havechanged so as to have a relationship as shown in FIG. 8D, the signalφVPS fed to the source of the MOS transistor T1 is turned to a highlevel. This causes the potentials in the MOS transistor T1 to have arelationship as shown in FIG. 8E, and thus causes those potentials to bereset to their original levels. After the potentials in the MOStransistor T1 have been reset to their original levels in this way, thevoltage of the signal φD is turned to a low level so that the capacitorC2 is discharged and thereby the potential at the node “a” is reset toits original level. Thereafter, the voltage of the signal φD is turnedback to a high level in preparation for image sensing.

In this way, resetting is achieved by manipulating the voltage fed tothe source of the MOS transistor T1 of which the drain is electricallyconnected to the photodiode serving as a photosensitive element. Thishelps improve the response of the individual pixels of the solid-stateimage-sensing device, and thereby makes it possible to performsatisfactory image shooting free from after-images even when a dimly-litsubject is shot or when a thus far brightly-lit subject has suddenlybecome dimly-lit.

The reading of the signal from each pixel may be achieved by means of aCCD (charge-coupled device). In that case, the transfer of an electriccharge to the CCD is achieved by providing a potential barrier with avariable potential that corresponds to the MOS transistor T3 shown inFIG. 3 or 6.

Second Example of the Pixel Configuration

FIG. 9 schematically shows the configuration of part of anothertwo-dimensional MOS-type solid-state image-sensing device embodying theinvention. In this figure, reference symbols G11 to Gmn represent pixelsthat are arranged in a two-dimensional array (in a matrix). Referencenumeral 2 represents a vertical scanning circuit, which scans lines(rows) 4-1, 4-2, . . . , 4-n sequentially. Reference numeral 3represents a horizontal scanning circuit, which reads out, sequentiallypixel by pixel in a horizontal direction, the signals fed from theindividual pixels to output signal lines 6-1, 6-2, . . . , 6-m as aresult of photoelectric conversion performed in those pixels. Referencenumeral 5 represents a power line. The individual pixels are connectednot only to the lines 4-1, 4-2, . . . , 4-n, to the output signal lines6-1, 6-2, . . . , 6-m, and to the power line 5 mentioned above, but alsoto other lines (for example clock lines and bias supply lines). Theseother lines, however, are omitted in FIG. 9, and are shown in individualembodiments of the invention shown in FIG. 11 and the following figures.

As shown in FIG. 9, for each of the output signal lines 6-1, 6-2, . . ., 6-m, a pair of N-channel MOS transistors Q1 and Q2 is provided. Here,a description will be given only with respect to the output signal line6-1 as their representative. The MOS transistor Q1 has its gateconnected to a direct-current voltage line 7, has its drain connected tothe output signal line 6-1, and has its source connected to adirect-current voltage VPSA line 8. On the other hand, the MOStransistor Q2 has its drain connected to the output signal line 6-1, hasits source connected to a signal line 9 serving as a final destinationline, and has its gate connected to the horizontal scanning circuit 3.

As will be described later, the pixels G11 to Gmn are each provided withan N-channel MOS transistor Ta that outputs a signal in accordance withthe photoelectric charge generated in each pixel. How this MOStransistor Ta is connected to the above-mentioned MOS transistor Q1 isshown in FIG. 10A. This MOS transistor Ta corresponds to a fourth MOStransistor T4 in the third, fourth, sixth, eighth, and ninthembodiments, and corresponds to a second MOS transistor T2 in the fifth,seventh, and tenth embodiments. Here, the direct-current voltage VPSAconnected to the source of the MOS transistor Q1 and the direct-currentvoltage VPDA connected to the drain of the MOS transistor Ta fulfill therelation VPDA>VPSA, where the direct-current voltage VPSA is equal to,for example, the ground-level voltage. In this circuit configuration,the signal from a pixel is fed to the gate of the upper-stage MOStransistor Ta, and a direct-current voltage DC is kept applied to thegate of the lower-stage MOS transistor Q1. Thus, the lower-stage MOStransistor Q1 is equivalent to a resistor or constant-current source,and therefore the circuit shown in FIG. 10A forms an amplifier circuitof a source-follower type. Here, it can safely be assumed that, as aresult of amplification, the MOS transistor Ta outputs a current.

The MOS transistor Q2 is controlled by the horizontal scanning circuit 3so as to function as a switching device. As will be described later, inall of the embodiments of the invention shown in FIG. 11 and thefollowing figures, within each pixel, another, i.e. a third, N-channelMOS transistor T3 functioning as a switch is provided. If this third MOStransistor T3 is illustrated explicitly, the circuit shown in FIG. 10Ahas, more precisely, a circuit configuration as shown in FIG. 10B.Specifically, the MOS transistor T3 is inserted between the MOStransistor Q1 and the MOS transistor Ta. Here, the MOS transistor T3serves to select a row, and the MOS transistor Q2 serves to select acolumn. It is to be noted that the circuit configurations shown in FIGS.9, 10A, and 10B are common to the third to tenth embodiments of theinvention described hereafter.

The circuit configuration shown in FIGS. 10A and 10B permits the signalto be output with a high gain. Accordingly, even in a case where thephotoelectric current generated in a photosensitive element is convertednatural-logarithmically to obtain a wider dynamic range and thus theoutput signal obtained is comparatively low, this amplifier circuitamplifies the signal so as to make it sufficiently high and thus easierto process in the succeeding signal processing circuit (not shown).Here, the MOS transistor Q1 that serves as the load resistor of theamplifier circuit is provided within each pixel; however, suchtransistors may be provided, instead, one for each of the output signallines 6-1, 6-2, . . . , 6-m, i.e. one for each of the groups of pixelsthat individually constitute columns, with the pixels constituting eachcolumn collectively connected to one of the output signal lines 6-1,6-2, . . . , 6-m. This helps reduce the number of load resistors orconstant-current sources required, and thus reduce the area occupied bythe amplifying circuits on a semiconductor chip.

Third Embodiment

Next, a third embodiment of the invention, which is applicable to eachpixel of the second example of the pixel configuration shown in FIG. 9,will be described with reference to the drawings. FIG. 11 is a circuitdiagram showing the configuration of each pixel of the solid-stateimage-sensing device of this embodiment. It is to be noted that suchelements, signal lines, and others as are used for the same purposeshere as in the pixel shown in FIG. 3 are identified with the samereference symbols, and their detailed descriptions will be omitted.

As shown in FIG. 11, in this embodiment, as compared with the pixelshown in FIG. 3, there are provided additionally a fourth MOS transistorT4 having its gate connected to the node “a” for performing currentamplification in accordance with the voltage at the node “a”, and afifth MOS transistor T5 for initializing the potential at the node “a”.The source of the MOS transistor T3 is connected to the output signalline 6 (this output signal line 6 corresponds to the output signal lines6-1, 6-2, . . . 6-m shown in FIG. 9). Just like the MOS transistors T1to T3, the MOS transistors T4 and T5 are both N-channel MOS transistors,and have their back gates grounded.

A direct-current voltage VPD is applied to the drain of the MOStransistor T4, and a signal φV is fed to the gate of the MOS transistorT3. A direct-current voltage VRB2 is applied to the source of the MOStransistor T5, and a signal φVRS is fed to the gate of the same MOStransistor T5. The direct-current voltage VPD is applied also to thedrain of the MOS transistor T2. In this embodiment, the MOS transistorsT1 to T3 and the capacitors C1 and C2 operate in the same manner as inthe first embodiment (FIG. 3) so as to output an electric signal (outputsignal) obtained by converting the amount of incident lightlogarithmically.

(1) How to Convert the Light Incident on the Individual Pixels into anElectric Signal

In this embodiment, as in the first embodiment, by turning the voltageof the signal φVRB to a low level so that the MOS transistor T1 operatesin a subthreshold region, it is possible to output to the output signalline 6 an output signal obtained by logarithmically converting thephotoelectric current output from the photodiode PD in accordance withthe amount of incident light. Now, a description will be given of howthe individual circuit elements of the pixel shown in FIG. 11 operatewhen the output signal is produced by converting the photoelectriccurrent logarithmically.

When light enters the photodiode PD, a photoelectric current isgenerated therein, and, due to the subthreshold characteristics of MOStransistors, a voltage having the value obtained by converting thephotoelectric current natural-logarithmically appears at the gates ofthe MOS transistors T1 and T2. This voltage causes a current to flowthrough the MOS transistor T2, and, as a result, an electric charge thatis equivalent to the value obtained by converting the integral of thephotoelectric current natural-logarithmically is accumulated in thecapacitor C2. That is, a voltage proportional to the value obtained byconverting the integral of the photoelectric currentnatural-logarithmically appears at the node “a” between the capacitor C2and the source of the MOS transistor T2. Here, the MOS transistors T3and T5 are off.

Next, the pulse signal φV is fed to the gate of the MOS transistor T3 toturn this MOS transistor T3 on. This causes a current proportional tothe voltage applied to the gate of the MOS transistor T4 to be fedthrough the MOS transistors T3 and T4 to the output signal line 6. Here,the voltage applied to the gate of the MOS transistor T4 is equal to thevoltage appearing at the node “a”, and therefore the current thus fed tothe output signal line 6 has the value obtained by converting theintegral of the photoelectric current natural-logarithmically. In thisway, a signal (output current) proportional to the logarithm of theamount of incident light is read, and thereafter the MOS transistor T3is turned off.

(2) How to Reset the Individual Pixels

Now, how resetting is achieved in the pixel having the circuitconfiguration shown in FIG. 11 will be described with reference to thedrawings. FIG. 12 is a timing chart of the signals fed to the signallines connected to the individual circuit elements constituting eachpixel during resetting. As described under (1) above, in the pixelhaving the circuit configuration shown in FIG. 11, when the pulse signalφV is fed to the gate of the MOS transistor T3, an electric signal(output signal) obtained by converting the amount of incident lightlogarithmically is output to the output signal line 6. After this outputsignal has been output, the pulse signal φV turns to a low level, andresetting starts. In this embodiment, the relationship of the potentialsin the MOS transistor T1 during resetting is the same as in the firstembodiment, i.e. as shown in FIGS. 5B to 5E. Now, how this resetting isachieved will be described with reference to FIGS. 5A to 5E and 12.

As described above, after the output signal has been output as a resultof the pulse signal φV being fed to the gate of the MOS transistor T3,resetting starts. When resetting starts, as in the first embodiment, anegative electric charge flows into the MOS transistor T1 via the sourcethereof, causing the potentials in the MOS transistor T1 to have arelationship as shown in FIG. 5B.

Next, the voltage φVRB fed to the capacitor C1 is made higher so as tomake the gate voltage of the MOS transistor T1 higher. This causes thepotentials at the sub-gate region and drain of the MOS transistor T1 tobecome higher as shown in FIG. 5C. This increases the amount of negativeelectric charge that flows into the MOS transistor T1 via the sourcethereof, and thereby permits quick recombination of the positiveelectric charges accumulated at the gate and drain of the MOS transistorT1, at the gate of the MOS transistor T2, at the anode of the photodiodePD, and in the capacitor C1.

As a result, as shown in FIG. 5D, the potentials at the drain andsub-gate region of the MOS transistor T1 become lower. When thepotentials in the MOS transistor T1 have changed so as to have arelationship as shown in FIG. 5D, the voltage φVRB fed to the capacitorC1 is turned to a low level, and the gate voltage of the MOS transistorT1 is lowered. This causes the potentials at the drain and sub-gateregion of the MOS transistor T1 to have a relationship as shown in FIG.5E, and thus causes those potentials to be reset to their originallevels. After the potentials in the MOS transistor T1 have been reset totheir original levels in this way, the pulse signal φVRS is fed to thegate of the MOS transistor T5 so that the capacitor C2 is dischargedthrough the MOS transistor T5 and thereby the potential at the node “a”is reset to its original level.

Fourth Embodiment

Next, a fourth embodiment of the invention will be described withreference to the drawings. FIG. 13 is a circuit diagram showing theconfiguration of each pixel of the solid-state image-sensing device ofthis embodiment. It is to be noted that such elements, signal lines, andothers as are used for the same purposes here as in the pixel shown inFIG. 11 are identified with the same reference symbols, and theirdetailed descriptions will be omitted.

As shown in FIG. 13, in this embodiment, the initialization of thepotential at the capacitor C2, and thus at the node “a”, is achieved byfeeding the signal φD to the drain of the MOS transistor T2, and thusthe MOS transistor T5 is omitted. In other respects, each pixel of thisembodiment is configured in the same manner as in the third embodiment(FIG. 11). Here, while the signal φD is kept at a high level, thecapacitor C2 performs integration; while the signal φD is kept at a lowlevel, the electric charge accumulated in the capacitor C2 is dischargedthrough the MOS transistor T2 to make the voltage at the capacitor C2,and thus at the gate of the MOS transistor T4, approximately equal tothe low-level voltage of the signal φD (that is, the voltage is reset).In this embodiment, the omission of the MOS transistor T5 helps simplifythe circuit configuration.

In this embodiment, image sensing is achieved in the following manner.As in the third embodiment, the signal φVRB fed to the capacitor C1 isturned to a low level so that the MOS transistor T1 operates in asubthreshold region. Moreover, the signal φD is turned to a high levelso that an electric charge equivalent to the value obtained byconverting the integral of the photoelectric currentnatural-logarithmically is accumulated in the capacitor C2. Then, withpredetermined timing, the MOS transistor T3 is turned on so that acurrent proportional to the voltage applied to the gate of the MOStransistor T4 is fed through MOS transistors T3 and T4 to the outputsignal line 6.

On the other hand, resetting of the pixel is achieved by controlling thesignals with the timing shown in FIG. 4 as in the first embodiment.Specifically, as in the first embodiment, resetting starts after thefeeding of the pulse signal φV. Then, the voltage φVRB fed to thecapacitor C1 is made higher so as to make the gate voltage of the MOStransistor T1 higher. This increases the amount of negative electriccharge that flows into the MOS transistor T1 via the source thereof, andthereby permits, as in the first embodiment, quick recombination of thepositive electric charges accumulated at the gate and drain of the MOStransistor T1, at the gate of the MOS transistor T2, at the anode of thephotodiode PD, and in the capacitor C1.

Then, the voltage φVRB applied to the capacitor C1 is turned back to itsoriginal level so as to make the gate voltage of the MOS transistor T1lower and thereby reset the potentials at the drain and sub-gate regionof the MOS transistor T1 to their original levels. After the potentialsat the drain and sub-gate region of the MOS transistor T1 have beenreset to their original levels in this way, the voltage of the signal φDis turned to a low level so as to discharge the capacitor C2 and therebyreset the potential at the node “a” to its original level. Thereafter,the voltage of the signal φD is turned back to a high level inpreparation for image sensing.

Fifth Embodiment

Next, a fifth embodiment of the invention will be described withreference to the drawings. FIG. 14 is a circuit diagram showing theconfiguration of each pixel of the solid-state image-sensing device ofthis embodiment. It is to be noted that such elements, signal lines, andothers as are used for the same purposes here as in the pixel shown inFIG. 13 are identified with the same reference symbols, and theirdetailed descriptions will be omitted.

As shown in FIG. 14, in this embodiment, the direct-current voltage VPDis applied to the drain of the MOS transistor T2, and the capacitor C2and the MOS transistor T4 are omitted. That is, the MOS transistor T2has its source connected to the drain of the MOS transistor T3. In otherrespects, each pixel of this embodiment is configured in the same manneras in the fourth embodiment (FIG. 13).

In this pixel configured as described above, image sensing is achievedin the following manner. As in the fourth embodiment, the signal φVRBfed to the capacitor C1 is turned to a low level so that the MOStransistor T1 operates in a subthreshold region. By making the MOStransistor T1 operate in this way, a current having the valuenatural-logarithmically proportional to the photoelectric current flowsthrough the MOS transistor T2 as its drain current.

Then, the pulse signal φV is fed to the gate of the MOS transistor T3 toturn on this MOS transistor T3. This causes a current having the valuenatural-logarithmically proportional to the photoelectric current to befed through the MOS transistor T3, as its drain current, to the outputsignal line 6. At this time, the drain voltage of the MOS transistor Q1(FIG. 9), which is determined by the on-state resistances of the MOStransistors T2 and Q1 and the current flowing therethrough, appears asthe output signal on the output signal line 6. After this output signalhas been read, the MOS transistor T3 is turned off.

On the other hand, resetting of the pixel is achieved in the followingmanner. As in the fourth embodiment, resetting starts after the feedingof the pulse signal φV. Then, the voltage φVRB fed to the capacitor C1is made higher so as to make the gate voltage of the MOS transistor T1higher. This increases the amount of negative electric charge that flowsinto the MOS transistor T1 via the source thereof, and thereby permits,as in the first embodiment, quick recombination of the positive electriccharges accumulated at the gate and drain of the MOS transistor T1, atthe gate of the MOS transistor T2, at the anode of the photodiode PD,and in the capacitor C1.

Then, the voltage φVRB applied to the capacitor C1 is turned to a lowlevel so as to make the gate voltage of the MOS transistor T1 lower andthereby reset the potentials at the drain and sub-gate region of the MOStransistor T1 to their original levels. In this way, the potentials inthe MOS transistor T1 are reset to their original levels to make thepixel ready for image sensing again.

In this embodiment, it is not necessary to perform integration of thephotoelectric signal by the use of a capacitor C2 as performed in thefourth embodiment described above, and thus no time is required for suchintegration, nor is it necessary to reset the capacitor C2. This ensuresaccordingly faster signal processing. Moreover, as compared with thefourth embodiment, the capacitor C2 and the MOS transistor T4 can beomitted, and this helps further simplify the circuit configuration andreduce the pixel size.

Sixth Embodiment

Next, a sixth embodiment of the invention will be described withreference to the drawings. FIG. 15 is a circuit diagram showing theconfiguration of each pixel of the solid-state image-sensing device ofthis embodiment. It is to be noted that such elements, signal lines, andothers as are used for the same purposes here as in the pixel shown inFIG. 13 are identified with the same reference symbols, and theirdetailed descriptions will be omitted.

As shown in FIG. 15, in this embodiment, the drain and gate of the MOStransistor T1 are not connected together directly as in the fourthembodiment (FIG. 13), but are connected to each other through a sixthMOS transistor T6. This MOS transistor T6 has its drain connected to thedrain of the MOS transistor T1, and has its source connected to the nodebetween the gate of the MOS transistor T1 and the capacitor C1.Moreover, a signal φS is fed to the gate of the MOS transistor T6, and asignal φVPS is fed to the source of the MOS transistor T1. In thisembodiment, the signal φVPS is a ternary signal that takes one of threepredetermined levels, i.e. a low level, a high level, or an intermediatelevel, at a time. Here, a high level is, for example, a voltageapproximately equal to the direct-current voltage VPD, a low level is,for example, a voltage equal to the ground level, and an intermediatelevel is a voltage that is intermediate between those two voltages andthat causes the MOS transistor T1 to operate in a subthreshold region.An intermediate level is, for example, a voltage approximately equal tothe direct-current voltage VPS.

(1) How to Convert the Light Incident on the Individual Pixels into anElectric Signal

In this pixel configured as described above, image sensing is achievedin the following manner. First, the signal φS is turned to a high levelto bring the MOS transistor T6 into a conducting state, Moreover, thesignal φVPS is turned to an intermediate level so as to bias the MOStransistor T1 in such a way that the MOS transistor T1 operates in asubthreshold region. Furthermore, the signal φVRB fed to the capacitorC1 is turned to a low level. When the levels of these signals are set asdescribed above, the pixel of this embodiment has a circuitconfiguration quite similar to that of the fourth embodiment (FIG. 13).

In this state, when the signal 1D is turned to a high level, an electriccharge equivalent to the value obtained by converting the integral ofthe photoelectric current natural-logarithmically is accumulated in thecapacitor C2. Then, with predetermined timing, the MOS transistor T3 isturned on so that a current proportional to the voltage applied to thegate of the MOS transistor T4, i.e. the voltage at the node “a”, is fedthrough the MOS transistors T3 and T4 to the output signal line 6. Afterthis current signal obtained by converting the photoelectric currentnatural-logarithmically has been output in this way, the MOS transistorT3 is turned off.

(2) How to Reset the Individual Pixels

Now, how the resetting of the pixel is achieved will be described withreference to the timing chart shown in FIG. 16 and the potentialtransition diagrams shown in FIGS. 17B and 17C. In FIGS. 17B and 17C,the arrow at the left indicates the direction in which the potentialincreases. FIG. 17A is a sectional view, like FIGS. 5A and 8A, showingthe structure of the photodiode PD and the MOS transistor T1. Resettingstarts after the feeding of the pulse signal φV. First, the signal φS isturned to a low level to bring the MOS transistor T6 into anon-conducting state, and then the signal φVRB is turned to a highlevel. In this state, when the voltage of the signal φVPS is turned to alow level, the potentials in the MOS transistor T1 come to have arelationship as shown in FIG. 17B, causing a negative electric charge toflow into the MOS transistor T1 via the source thereof.

After this negative electric charge E having flown into the MOStransistor T1 has been accumulated therein as shown in FIG. 17B, thesignal φVPS is turned to a high level approximately equal to thedirect-current voltage VPD. This makes the potential at the source ofthe MOS transistor T1 higher than the potential at the sub-gate regionthereof, and thus causes part of the accumulated negative electriccharge E to flow out of the MOS transistor T1 via the source thereof. Asa result, as shown in FIG. 17C, now the negative electric charge E1remains accumulated at the drain of the MOS transistor T1, at the anodeof the photodiode PD, and at the gate of the MOS transistor T2. Now thatthe negative electric charge E1 remains accumulated in this way, thegate voltage of the MOS transistor T2 is determined by the negativeelectric charge E1. Since the negative electric charge E1 is determinedby the threshold voltage between the drain and gate of the MOStransistor T1, the gate voltage of the MOS transistor T2 is determinedby the threshold voltage of the MOS transistor T1.

With this state maintained, the signal φD is turned to a low level so asto reset the capacitor C2 once. Then, the signal φD is turned back to ahigh level so that the capacitor C2 is charged with a current sampled bythe gate voltage of the MOS transistor T2. Then, by feeding the pulsesignal φV, the voltage appearing at the node “a” as a result of thecapacitor C2 being charged in this way is subjected to currentamplification by the MOS transistor T4 and is then fed, as the outputsignal, through the MOS transistor T3 to the output signal line 6.

The detection of variations in sensitivity itself takes only a veryshort time, and therefore, in the process described above, thephotodiode does not necessarily have to be prevented from exposure tolight during the detection of variations. However, it is preferable thatthe photodiode be prevented at least from exposure to light so intenseas to affect the detection of variations in threshold values. Althoughit is preferable to keep the photodiode in a dark state, it suffices tokeep the photodiode in ambient light as long as there is no risk ofexposure to extremely intense light. This prevents the photodiode fromgenerating an unnecessary electric signal, and thereby makes it possibleto detect variations more accurately. This applies to all of theembodiments described hereafter.

After the signal having the value proportional to the threshold voltageof the MOS transistor T1, of which the variation leads to variations insensitivity of the individual pixels, has been output in this way, thesignal φVPS is turned to a low level and the signal φVRB is turned to anintermediate level. Subsequently, the signal φS is turned to a highlevel to bring the MOS transistor T6 into a conducting state, and then,as in the fourth embodiment (FIG. 13), the signal φVRB is turned to ahigh level and then back to a low level so as to reset the MOStransistor T1 quickly. Then, by turning the signal φD to a low level andthen back to a high level, the capacitor C2 is reset.

In this way, according to the pixel configuration of this embodiment,after image sensing by the individual pixels, it is possible to acquire,as compensation data for correcting the outputs from the individualpixels, signals proportional to the threshold voltage of the MOStransistor of the individual pixels, of which the variation leads tovariations in sensitivity of the individual pixels. For example, notonly is the image data obtained from the individual pixels during imagesensing output to the succeeding circuit so as to be stored pixel bypixel in a memory provided therein, but also the current proportional tothe threshold voltage of the MOS transistor of one pixel after anotheris output serially by way of the signal line 9 shown in FIG. 9 to thesucceeding circuit so as to be stored pixel by pixel as compensationdata in another memory provided therein. Then, by correcting pixel bypixel the image data with the corresponding compensation data, it ispossible to eliminate components resulting from variations insensitivity of the individual pixels from the output signal. A practicalexample of how this compensation is achieved is shown in FIG. 36 andwill be described later.

Seventh Embodiment

Next, a seventh embodiment of the invention will be described withreference to the drawings. FIG. 18 is a circuit diagram showing theconfiguration of each pixel of the solid-state image-sensing device ofthis embodiment. It is to be noted that such elements, signal lines, andothers as are used for the same purposes here as in the pixel shown inFIG. 15 are identified with the same reference symbols, and theirdetailed descriptions will be omitted.

As shown in FIG. 18, in this embodiment, the voltage VPD is applied tothe drain of the MOS transistor T2, and the capacitor C2 and the MOStransistor T4 are omitted. In other respects, each pixel of thisembodiment is configured in the same manner as in the sixth embodiment(FIG. 15).

Thus, this embodiment is to the sixth embodiment what the fifthembodiment (FIG. 14) is to the fourth embodiment (FIG. 13). Accordingly,the photodiode PD, the MOS transistors T1, T2, T3, and T6, and thecapacitor C1 operate in the same manner as in the sixth embodiment toachieve image sensing and resetting. The output signal is obtained, asin the fifth embodiment, as a result of the current flowing out of theMOS transistor T2 being fed, as the output current, through the MOStransistor T3 to the output signal line 6. At this time, the drainvoltage of the MOS transistor Q1 (FIG. 9), which is determined by theon-state resistances of the MOS transistors T2 and Q1 and the currentflowing therethrough, appears as the output signal on the output signalline 6. After this output signal has been read, the MOS transistor T3 isturned off.

Eighth Embodiment

Next, an eighth embodiment of the invention will be described withreference to the drawings. FIG. 19 is a circuit diagram showing theconfiguration of each pixel of the solid-state image-sensing device ofthis embodiment. It is to be noted that such elements, signal lines, andothers as are used for the same purposes here as in the pixel shown inFIG. 11 are identified with the same reference symbols, and theirdetailed descriptions will be omitted.

As shown in FIG. 19, in this embodiment, a signal φVPS is fed to thesource of the MOS transistor T1, and the capacitor C1 is omitted. Inother respects, each pixel of this embodiment is configured in the samemanner as in the third embodiment (FIG. 11). The signal φVPS is a binarysignal that takes one of two predetermined levels, i.e. either a lowlevel or a high level, at a time. Here, a high level refers to a voltagethat is approximately equal to the direct-current voltage VPS and thatcauses the MOS transistor T1 to operate in a subthreshold region, and alow level refers to a voltage that is lower than that and that bringsthe MOS transistor T1 into a conducting state.

(1) How to Convert the Light Incident on the Individual Pixels into anElectric Signal

In this embodiment, as in the second embodiment (FIG. 6), by turning thevoltage of the signal φVPS to a high level so that the MOS transistor T1operates in a subthreshold region, it is possible to output to theoutput signal line 6 an output signal obtained by logarithmicallyconverting the photoelectric current output from the photodiode PD inaccordance with the amount of incident light. Now, a description will begiven of how the individual circuit elements of the pixel shown in FIG.19 operate when the output signal is produced by converting thephotoelectric current logarithmically.

When light enters the photodiode PD, a photoelectric current isgenerated therein, and, due to the subthreshold characteristics of MOStransistors, a voltage having the value obtained by converting thephotoelectric current natural-logarithmically appears at the gates ofthe MOS transistors T1 and T2. This voltage causes a current to flowthrough the MOS transistor T2, and, as a result, an electric charge thatis equivalent to the value obtained by converting the integral of thephotoelectric current natural-logarithmically is accumulated in thecapacitor C2. That is, a voltage proportional to the value obtained byconverting the integral of the photoelectric currentnatural-logarithmically appears at the node “a” between the capacitor C2and the source of the MOS transistor T2. Here, the MOS transistors T3and T5 are off.

Next, the pulse signal φV is fed to the gate of the MOS transistor T3 toturn this MOS transistor T3 on. This causes a current proportional tothe voltage applied to the gate of the MOS transistor T4 to be fedthrough the MOS transistors T3 and T4 to the output signal line 6. Here,the voltage applied to the gate of the MOS transistor T4 is equal to thevoltage appearing at the node “a”, and therefore the current thus fed tothe output signal line 6 has the value obtained by converting theintegral of the photoelectric current natural-logarithmically. In thisway, a signal (output current) proportional to the logarithm of theamount of incident light is read, and thereafter the MOS transistor T3is turned off.

(2) How to Reset the Individual Pixels

Now, how resetting is achieved in the pixel having the circuitconfiguration shown in FIG. 19 will be described with reference to thedrawings. FIG. 20 is a timing chart of the signals fed to the signallines connected to the individual circuit elements constituting eachpixel during resetting. As described under (1) above, in the pixelhaving the circuit configuration shown in FIG. 19, when the pulse signalφV is fed to the gate of the MOS transistor T3, an electric signal(output signal) obtained by converting the amount of incident lightlogarithmically is output to the output signal line 6. After this outputsignal has been output, the pulse signal φV turns to a low level, andresetting starts. In this embodiment, the relationship of the potentialsin the MOS transistor T1 during resetting is the same as in the secondembodiment, i.e. as shown in FIGS. 8B to 8E. Now, how this resetting isachieved will be described with reference to FIGS. 8A to 8E and 19.

As described above, after the output signal has been output as a resultof the pulse signal φV being fed to the gate of the MOS transistor T3,resetting starts. When resetting starts, as in the second embodiment, anegative electric charge flows into the MOS transistor T1 via the sourcethereof, causing the potentials in the MOS transistor T1 to have arelationship as shown in FIG. 8B.

Next, the signal φVPS fed to the source of the MOS transistor T1 isturned to a low level so as to bring the MOS transistor T1 into aconducting state as shown in FIG. 8C. This increases the amount ofnegative electric charge that flows into the MOS transistor T1 via thesource thereof, and thereby permits quick recombination of the positiveelectric charges accumulated at the gate and drain of the MOS transistorT1, at the gate of the MOS transistor T2, and at the anode of thephotodiode PD.

As a result, as shown in FIG. 8D, the potentials at the drain andsub-gate region of the MOS transistor T1 become lower. When thepotentials in the MOS transistor T1 have changed in this way, the signalφVPS fed to the source of the MOS transistor T1 is turned to a highlevel. This causes the potentials in the MOS transistor T1 to have arelationship as shown in FIG. 8E, and thus causes those potentials to bereset to their original levels. After the potentials in the MOStransistor T1 have been reset to their original levels in this way, thepulse signal φVRS is fed to the gate of the MOS transistor T5 so thatthe capacitor C2 is discharged through the MOS transistor T5 and therebythe potential at the node “a” is reset to its original level.

Ninth Embodiment

Next, a ninth embodiment of the invention will be described withreference to the drawings. FIG. 21 is a circuit diagram showing theconfiguration of each pixel of the solid-state image-sensing device ofthis embodiment. It is to be noted that such elements, signal lines, andothers as are used for the same purposes here as in the pixel shown inFIG. 19 are identified with the same reference symbols, and theirdetailed descriptions will be omitted.

As shown in FIG. 21, in this embodiment, the initialization of thepotential at the capacitor C2, and thus at the node “a”, is achieved byfeeding the signal φD to the drain of the MOS transistor T2, and thusthe MOS transistor T5 is omitted. In other respects, each pixel of thisembodiment is configured in the same manner as in the eighth embodiment(FIG. 19). Here, as in the second embodiment (FIG. 6), while the signalφD is kept at a high level, the capacitor C2 performs integration; whilethe signal φD is kept at a low level, the electric charge accumulated inthe capacitor C2 is discharged through the MOS transistor T2 to make thevoltage at the capacitor C2, and thus at the gate of the MOS transistorT4, approximately equal to the low-level voltage of the signal φD (thatis, the voltage is reset). In this embodiment, the omission of the MOStransistor T5 helps simplify the circuit configuration.

In this embodiment, image sensing is achieved in the following manner.As in the eighth embodiment, the signal φVPS fed to the source of theMOS transistor T1 is turned to a high level so that the MOS transistorT1 operates in a subthreshold region. Moreover, the signal φD is turnedto a high level so that an electric charge equivalent to the valueobtained by converting the integral of the photoelectric currentnatural-logarithmically is accumulated in the capacitor C2. Then, withpredetermined timing, the MOS transistor T3 is turned on so that acurrent proportional to the voltage applied to the gate of the MOStransistor T4 is fed through MOS transistors T3 and T4 to the outputsignal line 6.

On the other hand, resetting of the pixel is achieved by controlling thesignals with the timing shown in FIG. 7 as in the second embodiment.Specifically, as in the second embodiment, resetting starts after thefeeding of the pulse signal φV. Then, the signal φVPS fed to the sourceof the MOS transistor T1 is turned to a low level to bring the MOStransistor T1 into a conducting state. This increases the amount ofnegative electric charge that flows into the MOS transistor T1 via thesource thereof, and thereby permits, as in the second embodiment, quickrecombination of the positive electric charges accumulated at the gateand drain of the MOS transistor T1, at the gate of the MOS transistorT2, and at the anode of the photodiode PD.

Then, the signal φVPS fed to the source of the MOS transistor T1 isturned to a high level so as to reset the potentials in the MOStransistor T1 to their original levels. After the potentials in the MOStransistor T1 have been reset to their original levels in this way, thevoltage of the signal φD is turned to a low level so as to discharge thecapacitor C2 and thereby reset the potential at the node “a” to itsoriginal level. Thereafter, the voltage of the signal φD is turned backto a high level in preparation for image sensing.

Tenth Embodiment

Next, a tenth embodiment of the invention will be described withreference to the drawings. FIG. 22 is a circuit diagram showing theconfiguration of each pixel of the solid-state image-sensing device ofthis embodiment. It is to be noted that such elements, signal lines, andothers as are used for the same purposes here as in the pixel shown inFIG. 21 are identified with the same reference symbols, and theirdetailed descriptions will be omitted.

As shown in FIG. 22, in this embodiment, the direct-current voltage VPDis applied to the drain of the MOS transistor T2, and the capacitor C2and the MOS transistor T4 are omitted. That is, the MOS transistor T2has its source connected to the drain of the MOS transistor T3. In otherrespects, each pixel of this embodiment is configured in the same manneras in the ninth embodiment (FIG. 21).

In this pixel configured as described above, image sensing is achievedin the following manner. As in the ninth embodiment, the signal φVPS fedto the source of the MOS transistor T1 is turned to a high level so thatthe MOS transistor T1 operates in a subthreshold region. By making theMOS transistor T1 operate in this way, a current having the valuenatural-logarithmically proportional to the photoelectric current flowsthrough the MOS transistor T2 as its drain current.

Then, the pulse signal φV is fed to the gate of the MOS transistor T3 toturn on this MOS transistor T3. This causes a current having the valuenatural-logarithmically proportional to the photoelectric current to befed through the MOS transistor T3, as its drain current, to the outputsignal line 6. At this time, the drain voltage of the MOS transistor Q1(FIG. 9), which is determined by the on-state resistances of the MOStransistors T2 and Q1 and the current flowing therethrough, appears asthe output signal on the output signal line 6. After this output signalhas been read, the MOS transistor T3 is turned off.

On the other hand, resetting of the pixel is achieved in the followingmanner. As in the ninth embodiment, resetting starts after the feedingof the pulse signal φV. Then, the signal φVPS fed to the source of theMOS transistor T1 is turned to a low level so as to bring the MOStransistor T1 into a conducting state. This increases the amount ofnegative electric charge that flows into the MOS transistor T1 via thesource thereof.

As a result, as in the second embodiment, recombination of the positiveelectric charges accumulated at the gate and drain of the MOS transistorT1, at the gate of the MOS transistor T2, and at the anode of thephotodiode PD is achieved quickly. Then, the signal φVPS applied to thesource of the MOS transistor T1 is turned to a high level so as to resetthe potentials in the MOS transistor T1 to their original levels. Inthis way, the potentials in the MOS transistor T1 are reset to theiroriginal levels to make the pixel ready for image sensing again.

In this embodiment, it is not necessary to perform integration of thephotoelectric signal by the use of a capacitor C2 as performed in theninth embodiment described above, and thus no time is required for suchintegration, nor is it necessary to reset the capacitor C2. This ensuresaccordingly faster signal processing. Moreover, as compared with thethird embodiment, the capacitor C2 and the MOS transistor T4 can beomitted, and this helps further simplify the circuit configuration andreduce the pixel size.

In all of the first to tenth embodiments described thus far, the MOStransistors T1 to T6 provided within each pixel as active elements areall composed of N-channel MOS transistors; however, these MOStransistors T1 to T6 may be composed of P-channel MOS transistorsinstead. FIGS. 24, 25, and 28 to 35 show eleventh to twentiethembodiments, which are examples of modified versions of the first totenth embodiments described above in which P-channel MOS are used.Accordingly, in FIGS. 23 to 35, all the elements used and the voltagesapplied have the opposite polarities. For example, in FIG. 24 (theeleventh embodiment), the direct-current voltage VPD is connected to theanode of the photodiode PD, and the cathode thereof is connected to thedrain of the first MOS transistor T1 and to the gate of the second MOStransistor T2. The direct-current voltage VPS is applied to the sourceof the first MOS transistor T1.

When logarithmic conversion is performed in a pixel as shown in FIG. 24,the direct-current voltage VPS and the direct-current voltage VPDfulfill the relation VPS>VPD, thus an inverted relation as compared withthe case shown in FIG. 3 (the first embodiment). Moreover, the outputvoltage of the capacitor C2 is initially high, and drops as a result ofintegration. Moreover, when the third MOS transistor T3 is turned on, alow voltage is applied to the gate thereof. Furthermore, in theembodiments shown in FIG. 28 and the following figures (the thirteenthto twentieth embodiments), when the fifth or sixth MOS transistor T5 orT6 is turned on, a low voltage is applied to the gate thereof. Asdescribed above, in cases where P-channel MOS transistors are used,although how the voltages are applied and the elements are connecteddiffers partially, the circuits are configured substantially in the samemanner and operate basically in the same manner as in cases whereN-channel MOS transistors are used. Therefore, with respect to theeleventh to twentieth embodiments, only illustrations are given in FIGS.24, 25, and 28 to 35, and no descriptions will be given of theirconfiguration and operation.

FIG. 23 is a block circuit configuration diagram illustrating theoverall configuration of a solid-state image-sensing device havingpixels configured according to the eleventh or twentieth embodiment, andFIG. 26 is a block circuit configuration diagram illustrating theoverall configuration of a solid-state image-sensing device havingpixels configured according to one of the thirteenth to twentiethembodiments. As to FIGS. 23 and 26, such elements as are found also(i.e. as play the same roles as) in FIGS. 2 and 9 are identified withthe same reference symbols, and their descriptions will be omitted.Here, a brief description will be given of the configuration shown inFIG. 26. A P-channel MOS transistor Q1 and a P-channel MOS transistor Q2are connected to each of output signal lines 6-1, 6-2, . . . , 6-m thatare laid in the column direction. The MOS transistor Q1 has its gateconnected to a direct-current voltage line 7, has its drain connected tothe output signal line 6-1, and has its source connected to adirect-current voltage VPSA line 8.

On the other hand, the MOS transistor Q2 has its drain connected to theoutput signal line 6-1, has its source connected to a signal line 9serving as a final destination line, and has its gate connected to ahorizontal scanning circuit 3. Here, the MOS transistor Q1, togetherwith a P-channel MOS transistor Ta provided within each pixel,constitutes an amplifier circuit as shown in FIG. 27A. This MOStransistor Ta corresponds to the fourth MOS transistor T4 in thethirteenth, fourteenth, sixteenth, eighteenth, and nineteenthembodiments, and corresponds to the second MOS transistor T2 in thefifteenth, seventeenth, and twentieth embodiments.

Here, the MOS transistor Q1 serves as a load resistor orconstant-current source for the MOS transistor Ta. Accordingly, thedirect-current voltage VPSA connected to the source of this MOStransistor Q1 and the direct-current voltage VPDA connected to the drainof the MOS transistor Ta fulfill the relation VPDA<VPSA, where thedirect-current voltage VPDA is equal to, for example, the ground-levelvoltage. The MOS transistor Q1 has its drain connected to the MOStransistor Ta, and receives a direct-current voltage at its gate. TheP-channel MOS transistor Q2 is controlled by the horizontal scanningcircuit 3 so as to feed the output of the amplifier circuit to thesignal line 9 that serves as the final destination line. If the thirdMOS transistor T3 provided within each pixel is explicitly illustrated,the circuit shown in FIG. 27A has a circuit configuration as shown inFIG. 27B.

How to Correct Image Data with Pixels of the First to Tenth Embodiments

Now, with reference to the drawings, an embodiment of the invention willbe described in which a solid-state image-sensing device having pixelsconfigured according to one of the sixth, seventh, sixteenth, andseventeenth embodiments described above is employed in an image inputapparatus such as a digital camera.

The image input apparatus shown in FIG. 36 includes the followingcomponents. An objective lens 51 introduces the light from a subjectinto the image input apparatus. A solid-state image-sensing device 52outputs an electric signal in accordance with the amount of lightintroduced through the objective lens 51. A memory 53 receives theelectric signal (hereafter referred to as the “image data”) output fromthe solid-state image-sensing device 52 during image sensing and storesit temporarily. Another memory 54 receives the electric signal(hereafter referred to as the “compensation data”) output from thesolid-state image-sensing device 52 during resetting and stores ittemporarily. A compensation circuit 55 corrects the image data stored inthe memory 53 in accordance with the compensation data stored in thememory 54 by performing predetermined compensation calculation. A signalprocessor 56 performs predetermined processing on the image datacorrected in accordance with the compensation data by the compensationcircuit 55 and feeds out the processed image data. Here, the solid-stateimage-sensing device 52 has pixels configured as in one of the sixth,seventh, sixteenth, and seventeenth embodiments (FIGS. 15, 18, 31, and32).

This image input apparatus configured as described above operates asfollows. First, image sensing is performed and image data is outputpixel by pixel from the solid-state image-sensing device 52 to thememory 53. Then, after image sensing by the individual pixels, resettingis performed, and meanwhile, as described above, variations insensitivity of the individual pixels are detected and output, ascompensation data, to the memory 54. The image data stored in the memory53 and the compensation data stored in the memory 54 are fed pixel bypixel to the compensation circuit 55. The compensation circuit 55corrects the image data fed from the memory 53 in accordance with thecompensation data fed from the memory 54 by performing predeterminedcompensation calculation between the image data and compensation data ofcorresponding pixels. The image data thus corrected is fed to the signalprocessor 56, which performs predetermined processing on this image dataand then feeds it out. As the memories 53 and 54, there is no need touse frame memories, and it suffices to use line memories. This makesincorporation of these memories into the solid-state image-sensingdevice easy.

As described above, according to the present invention, it is possibleto realize an image-sensing apparatus provided with a solid-stateimage-sensing device that permits quick resetting of individual pixelsand that thus offers quick response in image sensing. This makes itpossible to obtain images free from after-images even when a dimly-litsubject is shot. Moreover, using MOS transistors to form active elementsmakes high-density integration possible, and thus makes it possible toform them on a single chip together with peripheral processing circuitssuch as A/D converters, digital system processors, and memories.

1. An image-sensing apparatus comprising: a plurality of pixels, thepixels each including: a photodiode, having two electrodes, thatproduces an electric signal in accordance with amount of incident light;a first MOS transistor having a first electrode and a gate electrodeconnected to one electrode of the photodiode and a second electrode; anda second MOS transistor having a first electrode, a second electrode,and a gate electrode connected to the first and gate electrodes of thefirst MOS transistor; a third MOS transistor having a first electrode towhich a direct-current voltage is applied, a second electrode, and agate electrode connected to the second electrode of the second MOStransistor, the third MOS transistor amplifying a signal output from thesecond electrode of the second MOS transistor; a fourth MOS transistorhaving a first electrode connected to the second electrode of the thirdMOS transistor, a second electrode connected to an output signal line,and a gate electrode connected to a line select line; and a controllerthat makes the individual pixels perform image sensing in such a waythat the electric signal output from the photodiode is convertednatural-logarithmically by feeding a first voltage to the secondelectrode of the first MOS transistor so as to make the first MOStransistor operate in a subthreshold region below a threshold valuethereof, wherein the controller resets the individual pixels by, in eachpixel, feeding a second voltage to the second electrode of the first MOStransistor so as to permit a larger current to flow through the firstMOS transistor than before feeding the second voltage thereto, and acapacitor having one end connected to the second electrode of the secondMOS transistor, the capacitor being reset through the second MOStransistor when a reset voltage is fed to the first electrode of thesecond MOS transistor.
 2. An image-sensing apparatus as claimed in claim1, further comprising: MOS transistors connected to the individualpixels by way of output signal lines so as to serve as load resistors orconstant-current sources.
 3. An image-sensing apparatus as claimed inclaim 1, wherein the pixels each further include a fifth MOS transistorhaving a first electrode connected to the second electrode of the secondMOS transistor, a second electrode to which a direct-current voltage isapplied, and a gate electrode; wherein the capacitor can be resetthrough the fifth MOS transistor when a reset voltage is fed to the gateelectrode of the fifth MOS transistor.
 4. An image-sensing apparatuscomprising: a plurality of pixels, the pixels each including: aphotodiode, having two electrodes, that produces an electric signal inaccordance with amount of incident light; a first MOS transistor havinga first electrode and a gate electrode connected to one electrode of thephotodiode and a second electrode; and a second MOS transistor having afirst electrode, a second electrode, and a gate electrode connected tothe first and gate electrodes of the first MOS transistor; a third MOStransistor having a first electrode to which a direct-current voltage isapplied, a second electrode, and a gate electrode connected to thesecond electrode of the second MOS transistor, the third MOS transistoramplifying a signal output from the second electrode of the second MOStransistor; a fourth MOS transistor having a first electrode connectedto the second electrode of the third MOS transistor, a second electrodeconnected to an output signal line, and a gate electrode connected to aline select line; a capacitor having one end connected to the secondelectrode of the second MOS transistor; and a controller that makes theindividual pixels perform image sensing in such a way that the electricsignal output from the photodiode is converted natural-logarithmicallyby feeding a first voltage to the second electrode of the first MOStransistor so as to make the first MOS transistor operate in asubthreshold region below a threshold value thereof, wherein thecontroller resets the individual pixels by, in each pixel, feeding asecond voltage to the second electrode of the first MOS transistor so asto permit a larger current to flow through the first MOS transistor thanbefore feeding the second voltage thereto.
 5. An image-sensing apparatusas claimed in claim 4, wherein the first, second, third and fourth MOStransistors are N-channel MOS transistors.
 6. An image-sensing apparatusas claimed in claim 4, wherein the first, second, third and fourth MOStransistors are P-channel MOS transistors.
 7. An image-sensing apparatuscomprising: a plurality of pixels, the pixels each including: aphotosensitive element, having two electrodes, that produces an electricsignal in accordance with amount of incident light; a first MOStransistor having a first electrode and a gate electrode connected toone electrode of the photosensitive element and a second electrode; anda second MOS transistor having a first electrode, a second electrode,and a gate electrode connected to the first and gate electrodes of thefirst MOS transistor; a third MOS transistor having a first electrode towhich a direct-current voltage is applied, a second electrode, and agate electrode connected to the second electrode of the second MOStransistor, the third MOS transistor amplifying a signal output from thesecond electrode of the second MOS transistor; a fourth MOS transistorhaving a first electrode connected to the second electrode of the thirdMOS transistor, a second electrode connected to an output signal line,and a gate electrode connected to a line select line; a capacitor havingone end connected to the second electrode of the second MOS transistor;and a controller that makes the individual pixels perform image sensingin such a way that the electric signal output from the photosensitiveelement is converted natural-logarithmically by feeding a first voltageto the second electrode of the first MOS transistor so as to make thefirst MOS transistor operate in a subthreshold region below a thresholdvalue thereof, wherein the controller resets the individual pixels by,in each pixel, feeding a second voltage to the second electrode of thefirst MOS transistor so as to permit a larger current to flow throughthe first MOS transistor than before feeding the second voltage thereto.8. An image-sensing apparatus as claimed in claim 7, further comprising:load resistors or constant-current sources connected to the outputsignal line, a total number of the load resistors or constant-currentsources being smaller than a total number of the pixels.
 9. Animage-sensing apparatus as claimed in claim 8, wherein the loadresistors or constant-current sources each comprise a resistivetransistor having a first electrode connected to the output signal lineand a second electrode and a control electrode connected to adirect-current voltage.
 10. An image-sensing apparatus as claimed inclaim 9, wherein the third transistor is an N-channel MOS transistor,and wherein a direct-current voltage applied to a first electrode of thethird transistor is higher than the direct-current voltage applied tothe second electrode of the resistive transistor.
 11. An image-sensingapparatus as claimed in claim 9, wherein the amplifying transistor is aP-channel MOS transistor; and wherein a direct-current voltage appliedto a first electrode of the amplifying transistor is lower than thedirect-current voltage applied to the second electrode of the resistivetransistor.